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Question DetailsAsked on 3/19/2017

how to fix efloressence from coming through slab were hyrostatic pressure is causing moisture spots?

we have a 20 year old house. we had a plumbing leak 1 year ago. after pulling up carpet, we had a concrete overlay
put on the floors. the overlay begin to darken and crack due to moisture and efloressence. . We put down a substance called dry crete but the eflorressence came back. What should we do.

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2 Answers


Hate to tell you, but someone led you astray if they said a silicate layer or an overlay would stop this. Basically speaking, "negative side" treatments like thjis do not work worth beans - you have to stop the water on the "positive" or wet side, which of course you cannot do without pulling the slab up.

You have water (leakage from pipe, leach field seepage, high groundwater table, rainfall from roof going down along the foundation and under it to the underside of the slab) wetting the slab - the water then moves through the slab by capillary action, evaporating at the surface and leaving behind mineral deposits - mostly lime from the concrete, which is the efflorescence you are seeing. Because the concrete is darkening, that almost certainly means the concrete is transmitting (wicking) free water, not just water vapor, so the water level is probably not far below the slab - or you are in a very low humidity area and the water is migrating to the surface in earnest and condensing on the underside of the cool slab, then wicking through it.

Depending on how thick the overlay was, if an inch and a half or two inches like a normal slab overlay is (as opposed to a skim coat overlay) had they put in a plastic liner under the overlay your problem might have beenn solved.

Five normal solutions - in no particular order

1) one is to seal the concrete with a penetrating sealer to limit the amount of flow-through - but if it is actually dark with moisture, it is unlikely to work (unless it dries out totally during the dry season as the water level goes down) because it will not penetrate the concrete and actually "seal" it when it is wet/damp. And like the "dry crete", none of these surface-applied products (as opposed to some added to the concrete mix products) really stop the movement of water - it will still find a way through cracks and along the surface of the aggregate and through the voids in the cement gel.

2) put a vapor barrier (plastic liner) under the concrete slab - obviously, this is only a solution when first placing (or you replace) the slab

3) put a vapor barrier over the slab - sealed at the basement walls, to contain the moisture and keep it from wetting the flooring. Not likely to work well in your case either for same reason - if actually looking damp you are likely to get actual water forming on the surface of the concrete as the humidity under the vapor barrier goes to 100%, which will commonly eventually go stagnant and start to stink under the flooring.

4) the permanent solution is to get rid of the water - put in gutters and downspouts to divert roof runoff away from the foundation area if that is the source of the water, or

5) put in drainage to lower the water table if high water levels (seasonal or perennial) is the issue. This latter can work unless you are in a floodplain or swampy area where the water table is high and you have highly permeable soils so you basically cannot reasonably remove the groundwater as fast as it comes in. If soil / water conditions are suitable, you have three ways to drain the water down - french drain around the outside of the foundation set well below foundation level to get the waer table well below the slab, draining away from the house to low ground (or if in low spot using a wet well with sump pump), putting in a deepish (typically 10 foot or so) sump with sump pump in the basement to draw the groundwater table down (if that is the source of the problem and soil is not so permeable that you would be pumping the world dry before it went down measureably), or putting in underdrains under the slab (cut in through it) leading to a sump pump. Obviously, each of these is easier to do during initial construction, but can be done after the fact as well and commonly are.

Note that your situation is different than the issue of free water flowing in around the slab perimeter or up through cracks and penetrations in the slab - your issue is not free water flowing under the foundation and upward (or through the foundation) so a normal-depth french drain (basically at the footer level) and foundation exterior waterproofing will not solve your issue - yours is almost certainly (since you said nothing about flooding) caused by the water table level being close below but not above the slab (close meaning within a few to as much as 10 feet depending on soil type - larger depth with silty and clayey and organic soils) and wetting the slab by capillary action.

You can find a lot of previous similar questions with answers in the Home > Basement Waterproofing link under Browse Projects, at lower left.

What exactly to do - depends on the situation. If only occurs during/immediately after rains or snowmelt conditions, then generally intercepting the water before it gets to the foundation is the solution - roof gutters and downspouts getting that bulk of water well away from the foundation to natural (or constructed) drainage leading away from the house, and maybe berming or swale or french drain construction in the yard to keep surface drainage runoff from getting near the house.

If high water table is the problem, then sometimes a deep (well below footer depth) french drain around the house (meaning excavation all around the foundation) will work if it can drain away to lower ground away from the house, but commonly that situation means you are in a low spot. French drains leading to a wet well outdoors, with a sump pump pumping the water out and to a natural drainage location away from the house can work at times - but commonly would require such a large pump that not only is it expensive to install and run, but of course when you lose power the water table will likely come up rapidly and wet the flooring again.

Indoors sump pump with or without underfloor drains work fairly well - though depending on soil type under the slab may have to be a couple of feet deep to stop the water transmission because the water will commonly wick up to 3-10 feet above the water table depending on how fine grained the soil is (finer grained, the higher it wicks). Pumping in a deeper sump without drain pipes under the slab only works in cases where the soil permeability if suitable for pumping but not real high, so you can create a drawdown or "cone of depression" under the house but are not trying to pump against a rapidly refilling cone of depression - and has to be carefully designed so it does not pull the fines out of the soil and cause settlement of the house. This "deep sump" or "drainage well" solution commonly fails to solve the problem because it is hard to predict exactly what the design parameters with the house already in place, so slab underdrains leading to a sump pump are more dependable a solution - but does mean cutting the slab severall places to install them - typically around the perimeter, and one to three lines down through the field of the slab in addition.

The bad news - unless you can solve the issue entirely by collecting and diverting the water away from the foundation (the exterior solutions), your slab is likely to still be dampish - meaning any flooring solution other than a very open-weave totally synthetic fabric carpet which can breathe as much as the moisture comes in is likely to have problems. This also presumes the ventilation/dehumidification in the basement is adequate to remove the moisture coming through the slab without creating high enough humidity (45-55% range) to start causing mold/mildew issues.

And on the painting/epoxy, etc situation - other than a very few underwater epoxies and urea finishes, they are not going to work well with a damp concrete - and will cost as much or more than a fairly high quality flooring, and if not truly designed for wet surface application, will generally bubble and peel fairly rapidly from the moisture underneath.

My recommendation in cases like this if not solveable by exterior water control - live with a bare concrete slab and clean the efflorescence with vinegar periodically. Or try putting down a vapor barrier sealed at the edges, and put a cheap readily removeable flooring over it - either cheap breathable segmentall (taped sections) area carpets which can be taken up for drying if it gets wet, or maybe a 100% plastic snap flooring which is designed to take apart and put back together, hoping that the water under the vapor barrier will not build up enough to stagnate. (Dusting the area with borax powder - not the Boraxo soap, but the laundry powder, before putting down the vapor barrier can help limit this problem).

One other thing I have seen done, but requires more than normal initial headroom - is putting in heavily retreated (with antifungal chemical) then air aged (to eliminate the odor issue) ground-contact timbers over the slab as new floor joists, with quite a high level of cross-ventilation (positibve ventilation fan system) designed into the subfloor system to remove the water vapor, then put your underlayment (treated plywood) and flooring over that - basically making a secondary (commonly 4x4 or 2x6 to provide enough height for good ventilation openings - or 4x4's blocked up on concrete pavers to keep them out of the water) subfloor system with moisture removal system which will still have the efflorescence but it will not affect the flooring. Needs care in designing the subfloor and ventilation system, and rubber padding to prevent "slap" against the slab when walked on - or stainless steel anchoring to the slab.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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